1. Field of the Invention
The present invention relates to a clutch device which is preferable when used as an electromagnetic clutch for controlling transmission of torque between rotating members or braking of the rotating members.
2. Description of Related Art
There has been known a braking mechanism in which rotation of the rotating members is restricted by a clutch controlled by excitation of an electromagnetic coil (see Japanese Patent Application Publication No. 2010-208584 (JP 2010-208584 A), for example)
The braking mechanism disclosed in JP 2010-208584 A includes a cam member which is rotated by a rotating member, a clutch plate arranged such that it is opposed to the cam member, cam balls interposed between the cam member and the clutch plate, an actuator which generates friction torque in the clutch plate and a return spring for generating urging force which separates the clutch plate from the actuator. The actuator includes an electromagnetic coil, a suction portion which accommodates the electromagnetic coil, and a frictional portion which is formed in the suction portion and makes frictional contact with the clutch plate.
When the electromagnetic coil is excited upon rotation of the rotating member and the clutch plate is brought into contact with the frictional portion due to its generated electromagnetic force, differential rotation is generated between the cam member and the clutch plate, so that this differential rotation generates cam thrust force with the cam balls. Due to this cam thrust force, the clutch plate is pressed against the frictional portion strongly so that the friction force between the clutch plate and the frictional portion is further increased. As a result, the braking mechanism turns into self-locking state, so that even if excitation of the electromagnetic coil is stopped after that, contact state between the clutch plate and the frictional portion is kept. That is, the braking mechanism is constructed in the form of a self-lock clutch.
In such a self-lock clutch, if it turns into an erroneous locking state in which it undergoes the self-locking due to some reason including vibration, the erroneous locking state is never released until the torque applied to the rotating member becomes smaller than a predetermined value. Thus, some fail safe operation is necessary. For example, a hybrid vehicle described in JP 2010-208584 A is so constructed that when the erroneous locking state occurs, supply of fuel to an engine is stopped and at the same time, a braking system is controlled to lower the velocity of the vehicle.
By the way, to prevent occurrence of the above-described erroneous locking state, it can be considered to increase the urging force of the return spring and then release the self-locking state with this urging force. However, if the urging force of the return spring is increased, a large electromagnetic force for activating the clutch correspondingly is required, thereby leading to increase in size of the electromagnetic coil or increase in consumption power. Further, depending on a magnitude of a torque applied to the rotating member, the self-locking state may not be released successfully by the urging force of the return spring.